Method of improving the crushing strength, impact resistance and compressibility of urea, and urea composition

ABSTRACT

A method of improving the crushing strength, impact resistance and the compressibility of urea granules by the addition of a compound to the molten urea, wherein the compound comprises both a polyvinyl compound and an organic molecule consisting of 1-10 carbon atoms and 1-10 polar organic groups.

The invention relates to a method of improving the crushing strength,impact resistance and the compressibility of urea granules by theaddition of a composition to the urea.

It is generally known that urea granulates are subject to crushingduring production, storage and transport. Up to 25 wt % of the ureagranulate may be crushed if the urea fertilizer is moved into a storagetank or storage ship at a relatively high temperature of e.g. more than40° C. Even though the urea granulate as such hardly absorb any moistureand are virtually non-susceptible to caking, high percentages of crushedgranules do lead to dust problems with a strong caking tendency.

It is also known that a number of chemical compositions can be used asan additive to improve the crushing strength, the caking tendency andthe resistance to moisture. Formaldehyde, hexamethylenetetramine andformaldehyde/urea condensate products are applied as crushing strengthimprovers, while the combination poly(vinylacetate)/surfactant (U.S.Pat. No. 4,812,158) is used to reduce the caking tendency. All theseadditives must either be added in relatively large quantities, or thetoxic characteristics make them difficult to handle. Moreover, the useof a surface active component as in poly(vinylacetate)/surfactant,results in increased foaming when the urea is used for industrialapplications, such as resin production, whilst formaldehyde derivativesare unsuitable in the case of melamine production. There is also a majordifference with respect to the use of additives. Both formaldehydederivatives and the compositions according to the invention are admixedto a urea melt. The combination polyvinylacetate/surfactant is, however,sprayed onto a preformed granule.

GB-A-1217106 describes a method of reducing the caking of urea by usinga poly(vinylalcohol) having a high molecular weight as an anticakingadditive. More in particular, according to this method, an aqueoussolution of the additive is admixed to an aqueous solution of urea.Preferentially, an amount of additive of from 0.005 to 5 wt %, based onthe weight of the urea, is admixed. The concentration of the aqueousurea solution is initially 80%, according to the example given; afteradmixture of the solution of the additive, concentration up to 95% takesplace at elevated temperature, after which the urea is allowed tocrystallize by cooling.

Patent WO 02/20471 discloses a method in which a combination of apolyvinyl compound and inorganic salts is admixed with a urea melt. Boththe crushing strength and the impact resistance of the resultinggranulate appeared to be improved compared to untreated urea. Inaddition, the compressibility of the urea was found to be decreasedsubstantially. The latter observation can be a great advantage as theurea granulates will be less prone to deformation upon storage. However,the introduction of inorganic salts, such as aluminium sulphate, in urearesults in a strong decrease of the pH upon dissolving the urea inwater. This can be a big disadvantage when using urea for technicalapplications, such as the production of resins.

It is therefore an object of the invention to provide a method by meansof which urea having improved crushing strength and impact resistance aswell as a low compressibility is obtained and by means of which theabovementioned drawbacks are overcome.

It has been found that the addition of a composition comprising both apolyvinyl compound and an organic molecule consisting of 1-10 carbonatoms and 1-10 polar organic groups to the urea melt leads to animprovement of both the hardness and the compressibility of the ureagranules formed. The organic components can be added either separatelyto the molten urea or together with the aqueous solution of thepolyvinyl compound. According to a preferred embodiment, the total addedamount of small organic molecules is at most 10% by weight based on thetotal amount of urea. The present application relates to an intimatemixture of urea, a polyvinyl compound and small molecules containinghydroxyl groups, resulting in increased hardness and decreasedcompressibility of the composition compared to untreated urea.Preferably the polyvinyl compound is a polyvinylalcohol, whereas thepreferred small polar organic molecule was found to be pentaerythritol.

To demonstrate the effect of the invention, a number of experiments werecarried out, employing methods which are representative for productionand for testing the quality of the urea granules produced.

Urea prills were prepared by mixing a urea melt with an aqueous solutionof the additive (of the concentrations specified in the experiments).The urea prills were formed by allowing molten urea droplets to descendin aliphatic oil having a viscosity of from 10 to 50 centipoise at 80°C. After crystallization the prills were taken from the oil and washedwith chloroform to remove the oil. The prills were sieved in order toobtain a particular size. The prills were dried in a fluidized bed for 1hour at about 40° C. They were collected and kept In an airtight flaskuntil both the crushing strength and the impact resistance weremeasured.

Urea pellets were produced by admixing an aqueous solution of theadditive (having the concentrations indicated in the experiments) with aurea melt consisting of 99.7 w/w % urea and 0.3 w/w % water. Thereafterthe urea pellets were formed by allowing the molten urea droplets tofall separately from a height of 1 cm onto a glass plate. Aftersolidification the pellets were scraped from the glass plate and thefines where removed by means of a sieve. The pellets were collected andkept in an airtight bottle until the compressibility was measured.

The crushing strength was determined via the IFDC S-115 method. Theimpact resistance was measured via the IFDC S-118 method.

The compressibility was determined as follows. A transparent round tubehaving an inner diameter of 3 cm was filled with 40 g urea pellets atroom temperature. Onto this a plunger was brought, through which apressure of about 600 kPa was exerted onto the sample. Directly afterthe application of the overpressure and again after 24 hours the heightof the urea column was measured. The relative difference in height,which is a measure for the compressibility, was calculated from thesetwo values (height (%)).

Experiment 1

A wide range of small polar organic molecules have been mixed withpolyvinylalcohol and added to the urea melt following the procedure asmentioned above. Crushing Impact Strength Resistance CompressibilityAdditive (N) (% fracture) (ΔHeight, %) Blank urea 13 75 16 1000 ppm of a12 wt % PVA 18 30 11 (>99% hydrolyzed) in H₂O 1000 ppm of a 12 wt % PVA21 35 9 and 1 m/m % Glycol solution in H₂O 1000 ppm of a 12 wt % PVA 2132 9 and 1 m/m % Glycerol solution in H₂O 1000 ppm of a 12 wt % PVA 1836 10 and 1 m/m % 1,4-butaandiol solution in H₂O 1000 ppm of a 12 wt %PVA 21 29 8 and 1 m/m % dimethylolurea solution in H₂O 1000 ppm of a 12wt % PVA 20 19 3 and 1 m/m % pentaerythritol solution in H₂O 1000 ppm ofa 12 wt % PVA 19 32 4 and 1 m/m % bishydroxymethylpropionic acidsolution in H₂O 1000 ppm of a 12 wt % PVA 17 39 5 and 1 m/m % tartaricacid solution in H₂O 1000 ppm of a 12 wt % PVA 14 46 9 and 1 m/m %citric acid solution in H₂O 1000 ppm of a 12 wt % PVA 16 55 7 and 1 m/m% lactic acid solution in H₂O 1000 ppm of a 12 wt % PVA 15 50 6 and 1m/m % succinic acid solution in H₂O 1000 ppm of a 12 wt % PVA 14 49 9and 1 m/m % gluconic acid solution in H₂OThis experiment shows that both the hardness and the compressibility ofurea granulates improved upon addition of a polyvinyl compound and smallpolar organic molecules. From these polar components pentaerythritol wasfound to give the most profound effect.Experiment 2

In order to select the best performing polyvinyl compound, differentpolymers were added to the urea melt in combination withpentaerythritol. Crushing Impact Strength Resistance CompressibilityAdditive (N) (% fracture) (ΔHeight, %) Blank urea 13 75 16 1000 ppm of a12 wt % PVA 18 30 11 (>99% hydrolyzed) in H₂O 1000 ppm of a 12 wt % PVA20 19 3 (>99% hydrolyzed) and 1 wt % pentaerythritol solution in H₂O1000 ppm of a 12 wt % PVA 19 28 9 (83% hydrolyzed) in H₂O 1000 ppm of a12 wt % PVA 19 19 4 (83% hydrolyzed) and 1 wt % pentaerythritol solutionin H₂O 1000 ppm of a 12 wt % 15 60 14 polymaleic acid solution in H₂O1000 ppm of a 12 wt % 17 51 9 polymaleic acid and 1 wt % pentaerythritolsolution in H₂OThis experiment demonstrates that the addition of a combination ofpolyvinyl compound and pentaerythritol to a urea melt results in ureagranulate with low compressibility and good hardness. The bestperformance was obtained when using a combination of polyvinylalcoholand pentaerythritol.Experiment 3

In order to find the optimum ratio between the polyvinyl compound andthe small polar organic compound, pentaerythritol was mixed withpolyvinylalcohol in different ratios and added to the urea melt.Crushing Impact Strength Resistance Compressibility Additive (N) (%fracture) (ΔHeight, %) Blank urea 13 75 16 1000 ppm of a 12 wt % PVA 1830 11 in H₂O 1000 ppm of a 12 wt % PVA 20 28 8 and 0.1 m/m %pentaerythritol solution in H₂O 1000 ppm of a 12 wt % PVA 21 25 5 and0.5 m/m % pentaerythritol solution in H₂O 1000 ppm of a 12 wt % PVA 2019 3 and 1.0 m/m % pentaerythritol solution in H₂O 1000 ppm of a 12 wt %PVA 19 21 3 and 1.5 m/m % pentaerythritol solution in H₂O 1000 ppm of a12 wt % PVA 20 23 4 and 2.0 m/m % pentaerythritol solution in H₂O 1000ppm of a 12 wt % PVA 18 26 6 and 3.0 m/m % pentaerythritol solution inH₂O 1000 ppm of a 12 wt % PVA 18 30 7 and 5.0 m/m % pentaerythritolsolution in H₂OWhen starting with 12 m/m % polyvinylalcohol, the best results areachieved in the presence of 0.5 to 1.5 m/m % pentaerythritol.Experiment 4

To determine the optimum dosage level for the optimum mixture ofexperiment 3, different amounts of this mixture were added to the ureamelt. Crushing Impact Strength Resistance Compressibility Additive (N)(% fracture) (ΔHeight, %) Blank urea 13 75 16 500 ppm of a 12 wt % PVA12 57 16 and 1.0 wt % pentaerythritol solution in H₂O 750 ppm of a 12 wt% PVA 15 42 9 and 1.0 wt % pentaerythritol solution in H₂O 1000 ppm of a12 wt % PVA 20 19 3 and 1.0 wt % pentaerythritol solution in H₂O 1250ppm of a 12 wt % PVA 21 24 4 and 1.0 wt % pentaerythritol solution inH₂O 1500 ppm of a 12 wt % PVA 20 21 4 and 1.0 wt % pentaerythritolsolution in H₂O 2000 ppm of a 12 wt % PVA 19 27 5 and 1.0 wt %pentaerythritol solution in H₂O 3000 ppm of a 12 wt % PVA 20 32 8 and1.0 wt % pentaerythritol solution in H₂OThis experiment shows that the optimum dosage of a mixture comprisingpolyvinylalcohol and pentaerythritol is between 750 and 1500 ppm byweight of urea to the urea melt.

1. A method of improving the crushing strength, impact resistance andthe compressibility of urea granules by the addition of a compound tothe molten urea, characterized in that the compound comprises both apolyvinyl compound and an organic molecule consisting of 1-10 carbonatoms and 1-10 polar organic groups.
 2. A method according to claim 1,characterized in that the polar organic groups are selected fromcarboxylic acid, hydroxyl, amine and/or amide groups.
 3. A methodaccording to claim 1, characterized in that the polar organic compoundconsists of between 2 and 5 carbon atoms.
 4. A method according to claim1, characterized in that the compound is pentaerythritol.
 5. A methodaccording to claim 1, characterized in that the amount polar organiccompound to be added in total is at most 1 wt %, based on the amount ofurea.
 6. A method according to claim 1, characterized in that the amountpolar organic compound to be added in total is between 5 and 100 ppm,based on the amount of urea.
 7. A method according to claim 1,characterized in that a polyvinyl additive is used of the generalformula (CHX—CHY)_(n), where n=4-10 000, and X and Y independently ofone another are selected from the group consisting of a hydrogen atomand a polar organic group.
 8. A method according to claim 7, wherein thesaid polar organic group is selected from a carboxylic acid group, anester group, a hydroxyl group, an amine group and an amide group.
 9. Amethod according to claim 7, wherein X is a hydrogen atom and Ysubstantially consists of a hydroxyl group.
 10. A method according toclaim 1, characterized in that at least 70%, preferably at least 95%, ofY consists of a hydroxyl group.
 11. A method according to claim 1,characterized in that an aqueous solution of the urea additive having aconcentration of from 0.5 to 25 wt % is used.
 12. A method according toclaim 1, characterized in that an aqueous solution of the urea additivehaving a concentration of from 1 to 20 wt % is used.
 13. A methodaccording to claim 1, characterized in that an aqueous solution of theurea additive having a concentration of from 100 to 10 000 ppm,preferably from 500 to 3 000 ppm, is used.
 14. Composition to be used inthe method according to claim 1 as a urea additive.